1. Field of the Invention
The present invention relates to an electronic endoscope that is capable of photographing an organ in a body, and it especially relates to a display of a spectral image obtained by emitting light having specific wavelengths.
2. Description of the Related Art
In an electronic endoscope, a color image is displayed by emitting a white light toward an object to be observed, and a spectral image with a color that is different from a normal image color can be also displayed by emitting light with a restricted bandwidth of wavelengths, such as ultraviolet rays, infrared rays, and so on. For example, a rotary filter in which an ND (Neutral Density) filter and an optical band-pass filter are formed circumferentially is arranged in an endoscope processor. The ND filter reduces the intensity or amount of all wavelengths of visible light equally when the visible light passes through the ND filter, whereas the band-pass filter blocks all light except for light having specific wavelengths. The rotary filter rotates in accordance to one field/frame interval. JP2011-200377A1 discloses such a rotating filter plate. Thus, image-pixel signals corresponding to normal white light and image-pixel signals corresponding to narrow-band light are read out from an image sensor alternately, and two images may be displayed on a screen in real time. An amount of light passing through the band-pass filter decreases compared to the same amount of light that enters the band-pass filter. To display a normal color image and a spectral image with the same brightness level, an amount of normal white light is reduced by a predetermined transmittance of the ND filter.
When the transmittance of the ND filter depends upon the amount of light that passes through the band-pass filter, the brightness of the normal observation image may not be sufficient for observation. And especially when an auto-brightness process is performed in accordance to a luminance level of the normal observation image, an aperture should be opened wider than usual, which could cause difficulty in the auto-brightness process.